So my 7-year-old Wolf Scout has his pinewood derby coming up. I want to convince him to build the car in a Kamm tail shape, like the Trek Speed Concept. Since I know even less about aerodynamics than I do about building a winning pinewood derby car (it’s all about the wheels but the secrets therein escape me), would this give the car any sort of aerodynamic advantage over the usual wedge-shape cars that show up?
I figure we could even use a countersinking drill bit to dimple the surface.
Just like the bike…at that speed…it is all rolling resistance. Wheels wheels wheels. Make sure the wheels are straight, smooth, the axles smooth, axles straight, hubs smooth…did I mention straight? And let your kid do everything else.
Black, with flames is the fastest. Weight is great, more is better in this context. Try to be right under the limit. Don’t you have to use the wheels from the kit? Try some Teflon tape around the axles (nails). But mostly, it’s about paint.
Can-o-worms now open!
First, I think the “advantage” of the Trek tube is mostly with side wind, yaw(n). No such issue with indoor derbies. In my experience with the PWD the most important thing is making the car go straight. Wheel alignment is key. Those kits come with grooves for the “axles” and they’re likely not aligned. Use tooth picks or whatever you can wedge in there. We used small finish nails.
As for aero I’d go for giro helmet shape. The long tail should help keep it going straight. Dimples by all means. Use a small ball peen hammer.
Graphite dry lube on the wheels and make sure they speed wobble to the point of whirrrrring.
Also VERY important; make sure the car is the max allowable weight. Add weights if you have to. Distribute them evenly.
He’s a smart kid and remembers how he placed last year when he was less able to do everything on his own, so in all liklihood he will not let me get anywhere near it or its design. But a dad can dream! Thanks for the tips on the axles.
A dad of two scouts last year was an auto body painter. The kids designed, cut, sanded, and built their cars, but he helped them with the painting, using auto paint and clear coats in a little paint booth he set up in his basement. Those cars were gorgeous, with a hard, clear finish and color so deep it was hard to believe they were made of wood.
My dad restored old cars as a hobby and we, too used auto paint. Glossy black (6 coats with fine sanding in between each coat) then 3 coats of clear. Car looks as good today as when we made it in 1979!
So my 7-year-old Wolf Scout has his pinewood derby coming up. I want to convince him to build the car in a Kamm tail shape, like the Trek Speed Concept. Since I know even less about aerodynamics than I do about building a winning pinewood derby car (it’s all about the wheels but the secrets therein escape me), would this give the car any sort of aerodynamic advantage over the usual wedge-shape cars that show up?
I figure we could even use a countersinking drill bit to dimple the surface.
Anyone?
In Pinewood Derby, aerodynamics mean MUCH less than:
(in order of importance IMHO)
Being at maximum weight allowedHaving the weight positioned as far back as possible to maximize the potential energy at the start line. Balance point of the car should be ~3/4" in front of the rear axle.Making sure the car tracks straight.Only 3 wheels touching. One of the front wheels should be “lifted”.Adequate wheel lubricationAxle and wheel bore polishing.Forget about aerodynamics and let your boy shape it however he wants…he’ll be a LOT more excited about it than if he let’s you talk him into something he really doesn’t want.
I second what Tom wrote. Put the weight as far back as possible because the heavy part of your car will be on the downward part of the track longer. You’ll have a hard time getting the balance point of the car farther back than 3/4" in front of the rear axle unless your seriously carve away the front and middle sections of the car.
Also, if your child shapes the block ensure that they do not carve away the area near where the nails/axles enter the wood block. The part of the wheel that rubs the wood block should contact a smooth flat section. The first year I built a car, when I was nine, I made it bullet shaped. It was super cool but It was the slowest car on the track because of the way the wheels contacted the cylindrical wood. I cried that night. That was when I realized that engineering and not aesthetics was going to win pinewood derby races. I did quite well in subsequent years.
Tom, I hadn’t heard of the lift a front wheel approach. I assume that the cars track ok, and don’t bump the center ridge this way? I can recall some cars jiggling from side to side toward the end of long tracks and slowing because they rubbed against the center ridge. I always assumed this was because too much nail/axle was exposed but now wonder about the height of the wheels.
I’d boar out the center of the wheels a bit with a mill and then insert a teflon bushing (or even better - Turcite) in the center. Make sure that the little bit that protrudes from the back of the wheel (the part that the axel goes through) is perfectly square and flush so that the wheels don’t drag on the body of the car. Then change the axels to a titanium-carbo-nitride-coated stainless steel pins that have been turned on a lathe. While you have the lathe out, chuck-up your wheels and turn them down so that the ‘treads’ are perfectly smooth and each wheel is exactly the same height.
I’d throw out the block of wood they give you and start with a nice piece of red oak. It’s harder and more dense, so it will take longer to work with, but you can get the shape lower without so much risk of breaking the car. I’d put the block back on the mill and drill some very small pilot holes for your axels to go into so that they are perfectly straight.
Put the weight in the middle of the car and make sure it hits the weight limit exactly. If you can get ahold of some ununubium to add mass, that will have better density than lead or copper - which most kids use - so it will have a smaller footprint in the car.
As far as the body shape goes, the lowest possible shape is the best - as long as you can put the weight in it somewhere. If you have solidworks and a CFD plug-in you should be able to sort out your precise design in less than 20 hours. If not you can just go with a proven shape: an airfoil bisected along it’s major axis with maximum height at about 40% of the length of the chord. But, as mentioned, aerodynamic drag at those speeds is pretty low. If you have access to the track before you build or if they have tech specs for your track somewhere, make sure that the underside of the car is as close to the guide strip on the center of the lane as reasonably possible.
Sand the surface of the car with something like 60 or 80 grit sand paper when you’re done and then just put a very little bit of wood stain on it - paint will mess up your aerodynamics as will varnish. Better than dimples in this instance.
Boron works better than graphite as a wheel lubricant.
Build a test track in your basement so that you can test various iterations.
This egregious blueprint for pinewood derby overkill has been brought to you by fasterisbetter* technical consulting. Enjoy.
In Pinewood Derby, aerodynamics mean MUCH less than:
(in order of importance IMHO)
Being at maximum weight allowedHaving the weight positioned as far back as possible to maximize the potential energy at the start line. Balance point of the car should be ~3/4" in front of the rear axle.Making sure the car tracks straight.Only 3 wheels touching. One of the front wheels should be “lifted”.Adequate wheel lubricationAxle and wheel bore polishing.Forget about aerodynamics and let your boy shape it however he wants…he’ll be a LOT more excited about it than if he let’s you talk him into something he really doesn’t want.
my dad helped me design one using almost all these principles and i won 3 years in a row plus the regional championship*
Thanks all for the suggestions on placing the weight and lifting one of the wheels.
As to the rest: maybe we can dispense with all of that and get our hands on a CNC machine, some billet aluminum alloy, and have at it. We can then use a plane to carve pieces for a pinewood veneer for highlights on the finished product. A laser might work well for insuring perfectly aligned axel holes, too.
My young scout is 7 and really good at Wii Lego Harry Potter. Programming a CNC machine should be a piece of cake, right?
In Pinewood Derby, aerodynamics mean MUCH less than:
(in order of importance IMHO)
Being at maximum weight allowedHaving the weight positioned as far back as possible to maximize the potential energy at the start line. Balance point of the car should be ~3/4" in front of the rear axle.Making sure the car tracks straight.Only 3 wheels touching. One of the front wheels should be “lifted”.Adequate wheel lubricationAxle and wheel bore polishing.Forget about aerodynamics and let your boy shape it however he wants…he’ll be a LOT more excited about it than if he let’s you talk him into something he really doesn’t want.
my dad helped me design one using almost all these principles and i won 3 years in a row plus the regional championship*
For his last 2 years, my boy took 2nd and 1st in his pack, and then DOMINATED the district championships in his age group and had the lowest average times in all of the preliminaries of any age groups. But, when they ran the “King of the Hill” races (the winner of each age group against each other) at the district both years, another boy’s car took a leap in performance after the allowed wheel re-lubing just prior to the “King of the Hill” round…whereas my boy’s car kept running the same times.
It’s funny…after the 2nd time having that happen in the “King of the Hill” round, and it being his last time doing the Derby, I thought he’d be pretty bummed…but, he actually told me that he was REALLY happy with how things turned out and wasn’t disappointed at all. I think he had more fun learning about potential energy being converted into kinetic energy and the concepts of friction reduction than in the actual competitions
The really funny thing is though…I swear his very first car which which we basically just “threw together” (i.e. not really paying attention to all of that stuff listed above since we hadn’t discovered it yet) was his overall fastest car. There’s something to be said for the “fluke” factor
